The primary advantage of an igniter according to the invention is that it is rain-safe. Previous types of fused igniters have ignited after arming even when struck by drops of water. This is a great disadvantage if one must open fire when it is raining as a great number of shells can be detonated long before they reach the target.
The general design of a fused impact igniter is based upon the fact that the igniter contains a conventionally designed rotor which, when the projectile is fired, strives to turn from a transport safety position to an armed position due to the centrifugal effect of rotation. To allow the rotor to turn, the striker pin, which in the transport safety position blocks the rotor, must be moved forward in the trajectory direction. This is prevented by a fuse formed like a locking washer and, located in the nose of the impact igniter. In earlier designs the fuse is located completely in the open at the furthest forward portion of the igniter. When the projectile is fired the surrounding air streams against and melts the fuse and the melted metal is carried away.
As soon as the fuse has melted, the rotor can then force the striker pin forward and turn to the armed position. The front end of the striker pin is then completely exposed in the opening which was originally covered by the fuse. A striker spring attempts to press the striker pin down into the ignition position. In order to retain the striker pin in the armed position until the projectile reaches its target, centrifugal locks comprised of balls, pendulums or similar devices have been used which have just been capable of retaining the striker pin in the armed position as long as the projectile has rotated sufficiently. When the projectile hits a target or its velocity has reduced sufficiently as with a miss, the centrifugal locking device has no longer been capable of functioning and the striker spring has forced the striker pin into the ignition position. As has already been pointed out, the front end of the striker pin has been completely unprotected from the front in the opening formerly covered by the fuse. This has meant that impact igniters of this type are activated if they happen to hit large rain drops which have directly contacted the front end of the striker pin.